The present invention relates to the transport of paper into and through a printer, particularly a matrix printer, having a barshaped printing platen, and wherein separate feed channels are provided for single sheet feeding as well as for endless sheet feeding.
The feeding, particularly the single sheet feeding of paper into a printer operating in conjunction with a platen, usually involves friction rollers operating and being constructed in pairs. Also, endless sheet feeding is usually carried out by means of tractors, or the like, which grip, for example, perforations arranged along an endless sheet and pull the endless sheet through the printer.
In addition, it is also know to have sensors provided in printers of this type which sensors monitor absence or presence of sheet stock in various locations in the printer in order to control various functions within the printer.
The feeding of paper into the printer does not merely involve just particular standardized sheets or webs of paper but various formats, various paper thicknesses, and, particularly stacks or bundles of paper for multi-copy printing, Leporello paper with and without edge perforation are also known. In accordance with one particular example, as shown, for example, in European patent application 99,958, corresponding to U.S. Pat. No. 4,579,471, paper is transported through sets of rollers, cooperating with a platen and arranged along the periphery of the platen roller. In addition, there is a brake provided in order to hold the paper against the platen. In accordance with another European patent application 99,957, corresponding to U.S. Patent application (Ser. No. 517,254, filed July 25, 1983), a second in line pressure roller set is provided for free-wheeling, and the platen roller itself can, in cases, be retracted slightly in order to avoid the formation of waves and slack in the paper. A taut i.e. tight abutment of the paper on the printing platen is necessary simply for the sake of clarity and precision of the characters being formed by the matrix printer. Also, the development of sound during printing will be reduced by taut positioning of the paper.
In accordance with newer experience, noise can be further attenuated through sound attenuation in the bearings for the printing platen, particularly if the platen is of bar-shaped, that is round in parts and non-circularly so. In particular, it was found in newer printers that it is of advantage to provide the platen with a flat and planar abutment surface against which printing obtains, as that will further improve the quality of printing. However, it can readily be seen that a platen with a flat printing surface changes basically the relationship as far as friction and adhesion is concerned as between the platen, on one hand, and the paper, on the other hand. There is no particular looping of the paper around the platen over a non-zero angle. This, in turn, means that in case one uses a printing platen with a planar printing surface, new elements have to be developed to still ensure lasting tight abutment of the paper against the platen, particularly during, as well as after threading paper into the printer.